Robert, you are correct, more or less,, it`s a big unknown quantity as far as illegal guns are concerned. there was an `amnesty` in SA, well,, a few amnesties ! ,they were popular with the police, as legal and illegal guns of all types were handed in for destruction, and most were resold by the cops there,, for pocket money,! that is a true story !!
however,,,
it is also reputed that the UK has upwards of 200,000 illegal guns,,,some put it at 4 million !!!! maybe not I think,,, but I did say an unknown quantity, :blind: if you bingle/bingle `how many illegal guns in UK,, quite a lot of stories,
not the original question of the OP, but relevant I feel !